An X-ray CT system is a device for measuring a computed tomographic image (hereinafter, referred to as “CT image”), and density of the image represents an X-ray attenuation coefficient (hereinafter, referred to as “attenuation coefficient”). According to a technological innovation of recent years, it is becoming possible to acquire a large quantity of CT images. As a result, in order to efficiently utilize the large quantity of CT images, it is of growing importance to have a function for automatically separating a composition of the object by the use of a computer. However, it is sometimes difficult to achieve such automatic separation of compositions (materials) which include X-ray attenuation coefficients being close to each other, as in the case of bone, organs, and vessels of a clinical material, for instance.
As a technique to solve this problem, there is suggested a dual energy imaging (hereinafter, referred to as “DE method”). A dual energy X-ray CT system which performs imaging according to the DE method irradiates the object with at least two types of X-rays having different energy spectra, and acquires CT images respectively based on the X-rays of two types or more. The two or more CT images being obtained show different values of the X-ray attenuation coefficient in the corresponding area of objects, respectively. This is because the X-ray attenuation coefficient of each composition depends on the energy spectrum of the X-ray being irradiated. Therefore, by using a map for separation obtained in advance, where the X-ray attenuation coefficient in an area of an object is associated with the composition, as to the X-rays of two types or more, it is possible to obtain an image which separates a composition (material) of the object area. As thus described, the use of the X-rays having two or more types of energy spectra allows the separation of the composition, though it has been difficult to separate the material by using the X-ray attenuation coefficient of the X-ray having only one type of energy spectrum. Such map for separation can be formed in advance by experiment or simulation.
The DE method is incorporated in an X-ray CT system used for a bomb test, which is installed for example at an airport, customs, or the like, and it is employed for separating a composition with reference to an atomic number and density information. In addition, the patent document 1 and the patent document 2 disclose that in a clinical-use X-ray CT system employed in a medical facility, separation is made between calcification in vessels and a bone, and between blood and soft plaque, or the like, by using image data of X-rays having two types of energy spectra. In the technique as disclosed in the non-patent document 1, a map for separation is formed by experiment or simulation, thereby separating calcium from a contrast medium.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2004-174253
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2004-065975
[Non-Patent Document 1]
Raz Carmi, et al., “Material Separation with Dual-Layer CT”, in Proc. Conf. Rec. IEEE Nuclear Science Symp., M03-367, 2005